Our main objective is to produce and detect chromosomal inversions and other specific chromosomal aberrations in mice by exposing sperm and spermatids to mutagens. We are continuing to identify the relationship of each aberration with genes on the respective chromosome as well as the chromosomal bands involved in the breakpoints. Each aberration will be maintained in homozygous condition or, if homozygous lethal, in balanced condition with a genetically marked normal chromosomal homologue. Our objective is to use these inversions as tools for mammalian studies in mutagenesis, cytyogenetics, linkage, meiotic and mitotic recombination, and dominant effects of recessive lethals and detrimentals. Over 20 inversions and 5 reciprocal translocations are being propagated, and studied. Combinations of inversions are being produced but one inversion has been used extensively to validate the first mammalian balanced lethal test to detect and maintain recessive lethals and detrimentals. Robertsonian chromosomes are being used in conjunction with inversions in an attempt to increase the power of such tests. A pericentric inversion has been induced this year and is being compared with paracentric inversions with respect to reproductive performance.